Take Vitamin D supplements? You'll want to read this.

New research on the benefits of vitamin D has implications for almost half of all U.S. adults, who are paying $600 million a year for vitamin D pills.

LONDON — Vitamin D supplements don't help prevent chronic diseases unrelated to the bones, according to a review of published research that challenges the prevailing wisdom held by proponents.

While scientific evidence supports the importance of vitamin D for bone health, its benefits in reducing the risk of diseases ranging from cancer to heart disease as shown in 290 observational studies were largely unconfirmed in 172 randomized controlled trials, experiments considered the gold standard for establishing causal links, according to the review published Friday in The Lancet Diabetes & Endocrinology medical journal.

The discrepancy in findings between the two types of studies suggests that low levels of vitamin D aren't a cause but a consequence of ill health, particularly inflammation linked to many diseases, according to the review's lead author, Philippe Autier, a professor at the International Prevention Research Institute in Lyon, France. The research has implications for almost half of all U.S. adults, who are paying $600 million a year for vitamin D pills.

“Associations between vitamin D and health disorders reported by investigators of observational studies are not causal,” Autier said in the published paper. “Low vitamin D could be the result of inflammatory processes involved in the occurrence and progression of disease.”

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In randomized controlled trials, scientists assign a treatment or placebo to study participants who don't know which one they're getting to tease out whether the treatment is causing the outcome. In observational studies, there are no such comparison groups.

The randomized trials on vitamin D showed little to no effect in lowering risk for heart disease, diabetes, infectious diseases and mood disorders, among other conditions. In contrast, the observational studies generally reported moderate to strong effects on these disorders.

In cancer, observational studies showed the supplements had a protective effect on colorectal cancer, an outcome that wasn't confirmed in two large randomized trials.

While randomized trials did show the supplement's benefit in reducing risk of death from any cause, those studies were mostly limited to elderly women living in nursing homes with low appetite and little exposure to sunlight, Autier said. It's unknown whether the same outcome would be observed in younger people, he said.

“These findings shouldn't be misinterpreted, that vitamin D has no value,” Duffy MacKay, vice president of scientific and regulatory affairs at the Council for Responsible Nutrition, the U.S. supplement industry's Washington-based trade group, said in a telephone interview. “This doesn't change the fact that we have recommended intake levels backed by the science around bone health, which is rock solid.”

The U.S. National Institutes of Health recommends 15 micrograms of daily vitamin D intake from ages 1 through 70. People older than 70 should consume 20 micrograms.

“This paper is a valuable contribution in the field of nutrition, but it has little to contribute to our problem in the U.K., where low levels of vitamin D result in hypocalcemic seizures in infants and bone disorders such as rickets,” said Colin Michie, chairman of the Nutrition Committee at the Royal College of Paediatrics and Child Health in London.

“It has been known for almost a century that vitamin D supplements given to those with deficient vitamin D levels results in improved bone health,” Michie said.

The review does have some limitations, such as the variable quality of the randomized trials, which weren't assessed by the authors.

Currently, there are five randomized controlled trials testing whether vitamin D supplements can reduce the risk of cancer, heart disease, diabetes, infections, declining brain functions, and fractures. Each study has at least 2,000 participants age 50 and older.

“The first results are not expected before 2017, but these studies have the potential to test our hypothesis,” Autier said in the published paper.

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